The goal of the proposed research is to understand the late events important for exocytosis, a process that underlies intercellular communication in the endocrine, exocrine and nervous systems. Studies will focus on the volume encompassing the plasma membrane and immediately adjacent secretory granules. The underlying hypothesis is that granule behavior and interactions immediately adjacent to the plasma membrane are highly regulated and important in determining the secretory response. The precise nature of the movement of granules to the plasma membrane and the plasma membrane docking step have been surmised, but have not been directly investigated. We are in a unique position to fill this gap. In the previous grant period we applied the powerful optical technique, total internal reflection fluorescence microscopy (TIRFM) to visualize granules immediately adjacentto the plasma membrane in living cells. We described for the first time fusion events from the point of view of a mobile granule membrane protein and discovered that granule motion is regulated by two regulators of exocytosis, ATP and micromolar Ca2+. We developed powerful analytical and biochemical tools to investigate granule and plasma membrane motion adjacent to the plasma membrane with TIRFM. We have also measured plasma membrane motions in resting cells using TIRFM, increased plasma membrane dynamics associated with individual exocytotic events using confocal microscopy, and have developed an intramolecular FRET probe to directly detect exocytotic SNARE interactions in living cells. We will apply these advances together with several novel, but well founded, TIRFM-based optical techniques to investigate in chromaffin cells: a) the regulation of granule motion, b) granule interaction with the plasma membrane, c) plasma membrane dynamics during exocytosis, and d) SNARE complex formation at exocytotic sites before and during exocytosis. These studies will further our knowledge of the key function in intercellular communication-the release of hormones or neurotransmitters. The results will be of fundamental importance in the understanding of neurological, endocrine and cardiovascular systems in health and disease.